1. Field of the Invention
This invention relates to a printing apparatus for making a printing plate by recording an image on the plate, and thereafter printing the image by feeding ink to the plate.
2. Description of the Related Art
In an ordinary conventional printing apparatus, a prepress process is carried out first to make a printing plate by exposing the plate placed in contact with a film having a binary black and white image recorded thereon. Then, the plate is loaded into the printing apparatus to carry out a printing process.
Recently, printing apparatus commonly called digital printers have been proposed, one such printer being capable of performing both the prepress process and printing process. The digital printers employ a xe2x80x9ccomputer-to-platexe2x80x9d system for forming an image on a printing plate by directly scanning and exposing the plate with laser beams or the like modulated with image signals.
In such a printing apparatus of the plate cylinder moving type, a plate cylinder having a printing plate mounted peripherally thereof must be rotated at a fixed rate in order for an image recording device to record an image on the plate. In an ordinary printing apparatus, generally, the plate cylinder has a cylinder gear disposed coaxially therewith to be rotatable by drive transmitted from a different cylinder. Thus, by using a drive system of such a printing apparatus to rotate the plate cylinder, an image may be recorded on the printing plate mounted peripherally of the plate cylinder.
Japanese Unexamined Patent Publication No. 2001-96712 discloses a printing apparatus employing a construction for rotating a plate cylinder by using the drive system of the printing apparatus at platemaking time. At this time, mechanical vibrations and load variations are avoided by operating a clutch to isolate loads of the drive system applied by a swing mechanism, paper gripping mechanism, ink feeding mechanism and the like.
When the image recording device records an image on the printing plate, the plate cylinder having a printing plate mounted peripherally thereof must be rotated at low speed and with a high degree of accuracy. When the drive system in the printing apparatus is utilized at this time as noted above, an inverter motor used in such drive system can rotate the plate cylinder at high speed, but is accompanied by considerable variations in the rotational accuracy. This baffles a high-accuracy image recording.
On the other hand, Japanese Unexamined Patent Publication No. 11-58671 (1999) discloses a printing apparatus in which, at platemaking time, a plate cylinder is moved from a printing position to a platemaking position, and then an image is recorded while rotating the plate cylinder by using a rotating mechanism of the traction type different from the drive system of the printing apparatus. This apparatus can avoid rotational variations of the plate cylinder to record the image with high precision, but requires a complicated construction for obtaining reproducibility of the position of the plate cylinder. Another disadvantage of the apparatus is that the movement of the plate cylinder is a time-consuming operation.
Japanese Unexamined Patent Publication No. 2000-280439 discloses a printing apparatus having a clutch provided between a plate cylinder gear and a plate cylinder. At platemaking time, the clutch is operated to break drive transmission between the plate cylinder and the drive system of the printing apparatus, and then an image is recorded while rotating the plate cylinder by using a motor other than a motor in the drive system of the printing apparatus. This apparatus can avoid rotational variations of the plate cylinder to record the image with high precision, but causes a misregistration due to a phase shift of the clutch at printing time. This poses a problem of requiring an additional device for attaining registration in the longitudinal direction.
The object of this invention is to provide a printing apparatus having a simple construction and yet capable of recording and printing images with high precision.
The above object is fulfilled, according to this invention, by a printing apparatus comprising a plate cylinder for supporting a printing plate as mounted peripherally thereof; a blanket cylinder having a blanket mounted peripherally thereof; an impression cylinder; an image recorder for recording an image on the printing plate mounted peripherally of the plate cylinder; an ink feeder for feeding ink to the printing plate mounted peripherally of the plate cylinder; a plate cylinder gear disposed laterally of the plate cylinder to be rotatable therewith; a blanket cylinder gear disposed laterally of the blanket cylinder to be rotatable therewith and meshable with the plate cylinder gear; an impression cylinder gear disposed laterally of the impression cylinder to be rotatable therewith and meshable with the blanket cylinder gear; a blanket cylinder gear moving mechanism for moving the blanket cylinder gear between a printing position for meshing with the plate cylinder gear, and a platemaking position separated from the plate cylinder gear; a first motor connected to the plate cylinder gear through the blanket cylinder gear for synchronously rotating the plate cylinder, the blanket cylinder and the impression cylinder when the blanket cylinder gear is in the printing position; and a second motor connected to the plate cylinder gear for rotating the plate cylinder when the blanket cylinder gear is in the platemaking position.
This printing apparatus includes the blanket cylinder gear moving mechanism for moving the blanket cylinder gear between a printing position for meshing with the plate cylinder gear, and a platemaking position separated from the plate cylinder gear. Thus, the printing apparatus, though simple in construction, can record and print images with high precision.
In a preferred embodiment, the blanket cylinder gear moving mechanism is arranged to move the blanket cylinder gear between the printing position for meshing with the impression cylinder gear and the plate cylinder gear, and the platemaking position for meshing with the impression cylinder gear but separated from the plate cylinder gear.
Preferably, the blanket cylinder gear is fixed laterally of the blanket cylinder, the blanket cylinder gear moving mechanism moving the blanket cylinder gear along with the blanket cylinder between the printing position and the platemaking position.
The second motor may be connected to the plate cylinder gear through a clutch, the plate cylinder having a rotary encoder for detecting rotational positions thereof at platemaking time.
Other features and advantages of the invention will be apparent from the following detailed description of the embodiments of the invention.